We analyze the performance of Geokinetic's oil shale retort 17 from offgas data. Retort 17 is a true in-situ combustion retort. The results predict that about 12100 metric tons of shale were processed in the retort. Using a total yield loss estimate of ~52% Fischer assay (based on gas chromatographic analysis of the oil), we predict a total oil production of 6.54 x 10sup5 litre (4115 bbl). This agrees to within 10% of the reported value of 6.00 x 10sup5 litre (3775 bbl). Based on the amount of oil loss due to combustion, we estimate that nearly all of the inlet oxygen (as air) must have been consumed in oil burning. This implies that the amount of char consumption by oxidation was small for this particular true in-situ retort.